1. Filed of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus configured to form toner images.
2. Description of Related Art
As an example of the past inventions relating to image forming apparatuses, for example, a developing device disclosed in Japanese Patent Laid-Open Publication No. 2001-312132 is known. Toner used in the developing device contains silica microparticles, strontium titanate and stearate as additives. The silica microparticles serve as mobile microparticles to heighten the fluidity of toner, and the strontium titanate serves as a polish. The stearate serves as an antifriction to prevent the silica microparticles from adhering to the surface of an image supporting member.
The developing device disclosed in Japanese Patent Laid-Open Publication No. 2001-312132 has a problem of causing the image supporting member to have a non-uniform film thickness in the main-scanning direction accompanied with use. FIG. 10 is a graph indicating the relation between the coefficient of friction and the point on the image supporting member. FIG. 11 is a graph indicating the relation between the film thickness and the point on the image supporting member. In FIG. 10, the y-axis indicates the coefficient of friction, and the x-axis indicates the point on the image supporting member in the main-scanning direction. In FIG. 11, the y-axis indicates the film thickness, and the x-axis indicates the point on the image supporting member in the-main scanning direction.
A developer is contained in the developing device, and the developer consists of toner particles and carrier particles, and further contains silica microparticles, strontium titanate and stearate as additives. The developing device has a first stirring/conveying member, a second stirring/conveying member and a developer supply member. The first stirring/conveying member and the second stirring/conveying member extend in the main-scanning direction. The first and second stirring/conveying members circulate the developer in the developing device while stirring the developer. The developer supply member is arranged to face the first stirring/conveying member. The developer supply member receives the developer from the first stirring/conveying member and develops an electrostatic latent image on the image supporting member with toner.
In the developing device, stearate separates from toner particles more easily than strontium titanate. Accordingly, while the developer is conveyed by the first stirring/conveying member, a large amount of stearate separates from toner particles around the upstream portion, with respect to the developer conveying direction, of the first stirring/conveying member. Therefore, the developer supported by the upstream portion, with respect to the developer conveying direction, of the developer supply member contains a relatively large amount of stearate. On the other hand, the developer supported by the downstream portion, with respect to the developer conveying direction, of the developer supply member contains a relatively small amount of stearate. Thus, the upstream portion of the developer supply member supplies toner containing a relatively large amount of stearate to the image supporting member, and the downstream portion of the developer supply member supplies a relatively small amount of stearate to the image supporting member.
When a relatively large amount of stearate adheres to the image supporting member, the coefficient of friction of the image supporting member is relatively high, and when a relatively small amount of stearate adheres to the image supporting member, the coefficient of friction of the image supporting member is relatively low. Therefore, as seen in FIG. 10, the coefficient of friction of the image supporting member in the upstream portion with respect to the developer conveying direction is relatively low, and the coefficient of friction of the image supporting member in the downstream portion with respect to the developer conveying direction is relatively high. At a time of cleaning the image supporting member with a cleaner blade, the cleaner blade scrapes the surface of the image supporting member more in the portion having a higher coefficient of friction. Therefore, with the developing device disclosed in Japanese Patent Laid-Open Publication No. 2001-312132, in the upstream portion with respect to the developer conveying direction, the surface of the image supporting member is not scraped much, and accordingly, the film thickness of the upstream portion of the image supporting member becomes relatively thicker. In the downstream portion with respect to the developer conveying direction, the surface of the image supporting member is scraped much, and accordingly, the film thickness of the upstream portion of the image supporting member becomes relatively thinner. Thus, the film thickness of the image supporting member becomes non-uniform in the main-scanning direction accompanied with use.